1. Field of the Invention
The present invention relates to a flexible printed circuit board having flexibility for use in wiring inside an electronic device or the like, and an electronic device incorporating the flexible printed circuit.
2. Description of the Related Art
Conventionally, flexible printed circuit boards having flexibility have frequently been used for wiring for conducting electrical signals inside an electronic device or the like. The flexible printed circuit board is structured with a copper foil pattern formed by etching a copper foil on a polyimide film serving as a base material and a translucent cover lay film compression-fixed and covered thereon. The copper foil pattern on the flexible printed circuit board may be formed not only for the purpose of forming wiring signals but also for the purposes other than conducting electrical signals, such as putting a product symbol, a fold index serving as a mark for use in folding the flexible printed circuit board, and the like.
Meanwhile, recently, a colored opaque cover lay film may be used as a cover lay film for covering the flexible printed circuit board. In such a case where the flexible printed circuit board is exposed outside the device, the copper foil pattern and the wiring circuit are no longer visible from outside the device by covering the printed circuit board with a colored (e.g., black) opaque cover lay film, thereby preventing the design from being impaired.
Regarding the symbols and the like formed on the flexible printed circuit board, there is disclosed a technique for providing a flexible printed circuit board (for example, see Japanese Laid-Open Patent Publication (Kokai) No. 9-205261), on a surface of which a photosensitive cover lay film is provided and marks such as letters and symbols are formed in an opening formed in the photosensitive cover lay film.
However, if the flexible printed circuit board is structured such that the copper foil pattern is covered with an opaque cover lay film, the product symbol as well as the fold index formed by the copper foil pattern on the flexible printed circuit board is also invisible. As a result, this structure causes a problem that assembly operators and manufacturers cannot visually see the fold index.
The technique disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 9-205261 has a problem that if a complicated character string is to be formed on the flexible printed circuit board, the character string cannot be formed in an opening of the cover lay film.
In a case of a structure in which an opening is formed in a cover lay film and letters or indices are formed in the opening portion by a copper foil pattern, when the flexible printed circuit board is bent and turned around inside an electronic device, the copper foil pattern having the letters or indices formed thereon is peeled off from the opening portion of the cover lay film. As a result, the peeled copper foil pattern may contact an electronic element inside the electronic device, resulting in a possible short circuit or the like.